Add more info to the inconsistent cardinality errors

prometheus/counter.go

@@ -136,7 +136,7 @@ func NewCounterVec(opts CounterOpts, labelNames []string) *CounterVec {
 	return &CounterVec{
 		metricVec: newMetricVec(desc, func(lvs ...string) Metric {
 			if len(lvs) != len(desc.variableLabels) {
-				panic(errInconsistentCardinality)
+				panic(makeInconsistentCardinalityError(desc.fqName, desc.variableLabels, lvs))
 			}
 			result := &counter{desc: desc, labelPairs: makeLabelPairs(desc, lvs)}
 			result.init(result) // Init self-collection.

prometheus/examples_test.go

@@ -282,7 +282,7 @@ func ExampleRegister() {
 	// taskCounter unregistered.
 	// taskCounterVec not registered: a previously registered descriptor with the same fully-qualified name as Desc{fqName: "worker_pool_completed_tasks_total", help: "Total number of tasks completed.", constLabels: {}, variableLabels: [worker_id]} has different label names or a different help string
 	// taskCounterVec registered.
-	// Worker initialization failed: inconsistent label cardinality
+	// Worker initialization failed: inconsistent label cardinality: expected 1 label values but got 2 in []string{"42", "spurious arg"}
 	// notMyCounter is nil.
 	// taskCounterForWorker42 registered.
 	// taskCounterForWorker2001 registered.

prometheus/gauge.go

@@ -147,7 +147,7 @@ func NewGaugeVec(opts GaugeOpts, labelNames []string) *GaugeVec {
 	return &GaugeVec{
 		metricVec: newMetricVec(desc, func(lvs ...string) Metric {
 			if len(lvs) != len(desc.variableLabels) {
-				panic(errInconsistentCardinality)
+				panic(makeInconsistentCardinalityError(desc.fqName, desc.variableLabels, lvs))
 			}
 			result := &gauge{desc: desc, labelPairs: makeLabelPairs(desc, lvs)}
 			result.init(result) // Init self-collection.

prometheus/histogram.go

@@ -165,7 +165,7 @@ func NewHistogram(opts HistogramOpts) Histogram {
 
 func newHistogram(desc *Desc, opts HistogramOpts, labelValues ...string) Histogram {
 	if len(desc.variableLabels) != len(labelValues) {
-		panic(errInconsistentCardinality)
+		panic(makeInconsistentCardinalityError(desc.fqName, desc.variableLabels, labelValues))
 	}
 
 	for _, n := range desc.variableLabels {

prometheus/labels.go

@@ -37,9 +37,22 @@ const reservedLabelPrefix = "__"
 
 var errInconsistentCardinality = errors.New("inconsistent label cardinality")
 
+func makeInconsistentCardinalityError(fqName string, labels, labelValues []string) error {
+	return fmt.Errorf(
+		"%s: %q has %d variable labels named %q but %d values %q were provided",
+		errInconsistentCardinality, fqName,
+		len(labels), labels,
+		len(labelValues), labelValues,
+	)
+}
+
 func validateValuesInLabels(labels Labels, expectedNumberOfValues int) error {
 	if len(labels) != expectedNumberOfValues {
-		return errInconsistentCardinality
+		return fmt.Errorf(
+			"%s: expected %d label values but got %d in %#v",
+			errInconsistentCardinality, expectedNumberOfValues,
+			len(labels), labels,
+		)
 	}
 
 	for name, val := range labels {
@@ -53,7 +66,11 @@ func validateValuesInLabels(labels Labels, expectedNumberOfValues int) error {
 
 func validateLabelValues(vals []string, expectedNumberOfValues int) error {
 	if len(vals) != expectedNumberOfValues {
-		return errInconsistentCardinality
+		return fmt.Errorf(
+			"%s: expected %d label values but got %d in %#v",
+			errInconsistentCardinality, expectedNumberOfValues,
+			len(vals), vals,
+		)
 	}
 
 	for _, val := range vals {

prometheus/summary.go

@@ -181,7 +181,7 @@ func NewSummary(opts SummaryOpts) Summary {
 
 func newSummary(desc *Desc, opts SummaryOpts, labelValues ...string) Summary {
 	if len(desc.variableLabels) != len(labelValues) {
-		panic(errInconsistentCardinality)
+		panic(makeInconsistentCardinalityError(desc.fqName, desc.variableLabels, labelValues))
 	}
 
 	for _, n := range desc.variableLabels {